This section is intended to provide a background or context for the invention disclosed below. The description to follow may include concepts that could be pursued, but have not necessarily been previously conceived, implemented or described. Therefore, unless otherwise explicitly indicated below, what is described in this section is not prior art to the description in this application and is not admitted to be prior art by inclusion in this section.
A filter is composed of a number of resonating structures and energy coupling structures which are arranged to exchange radio-frequency (RF) energy among themselves and input and output ports. The pattern of interconnection of these resonators to one another and to the input and output ports, the strength of these interconnections, and the resonant frequencies of the resonators determine the response of the filter.
During the design process for a filter, the arrangement of the parts, the materials from which the parts are made, and the precise dimensions of the parts are determined such that an ideal filter so composed will perform the desired filtering function. If a physical filter conforming exactly to this design could be manufactured, the resulting filter would perform exactly as intended by the designer.
However, in practice the precision and accuracy of manufacture of both the materials and the parts are limited, and the result is errors in resonant frequencies and coupling strengths, which, in turn, cause the filter response to differ from that predicted by an ideal filter model. Often, this departure from the ideal response is sufficiently large to bring the filter outside of its specification. As a result, it is desirable to include in the filter design some means of adjusting the resonator frequencies and couplings to bring the filter response within the specification.
A common way to accomplish this is to include, in or on the filter, tuning screws or other devices, such as are well known in the art. An alternative way often used in small ceramic monoblock filters is to remove selected portions of the metallization on the exterior of these filters, and possibly portions of ceramic as well, to perform the tuning.
Most filters are manufactured as completed units and, subsequent to manufacture, the tuning process is performed on the entire filter. Since adjustments on the filter may interact strongly with one another, the tuning procedure is often quite complicated, and requires a skilled operator.
An alternative tuning method is to build the resonator parts separately, tune them individually to a specification calculated for the separated parts from the ideal filter model, and then assemble them into the final filter. Since the individual parts are simple compared with the fully assembled filter, the tuning procedure for these individual parts can also be made very simple. This minimizes the need for skilled operators to tune the filters. This procedure also provides the benefit of either reducing or entirely eliminating the tuning process for the assembled filter.
In many cases, it is sufficient to adjust only the resonant frequencies of the resonator parts, because the manufacturing precision and accuracy for the resonator parts are good enough to bring the coupling strengths within the required range to allow the performance of the assembled filter to be within specification. In such cases, adjustment of the resonant frequencies is all that is required to tune the individual parts.
To facilitate pretuning of the individual parts, both methods of measurement of the frequencies and methods of adjustment of the frequencies are required. The present specification is concerned with the latter.
A tuning method may include the manipulation of a tuning device or structure included as part of the resonator, such as a tuning screw or deformable metal part. Alternatively, a method may comprise an operation performed on the resonator, such as the removal of material from a selected region. The method may also comprise a combination of these, or any other means or process which can alter the resonant frequencies of the resonator part.
A tuning physical adjustment (commonly abbreviated more simply as “adjustment”) can then be defined as one or more manipulations of tuning structures and/or one or more operations causing one or more of the resonant frequencies to be altered. For instance, such physical adjustment includes, but is not limited to, removal of material from a surface or face of a resonator component, drilling of holes in the resonator component, adjustments of screws in the resonator component, and/or denting of material covering the resonator component.
In cases where the parts include multimode resonators, the tuning methods for the parts will need to be capable of independently adjusting the resonant frequencies of the two or more modes of the resonator. For example, if the multimode resonator has three modes requiring independent adjustment, at least three independent tuning adjustments will be required. It is a common situation with multimode resonators that an individual adjustment causes more than one of the mode frequencies to change. As a result, there is no one-to-one correspondence between a single adjustment and a frequency change as there is in a single-mode filter.
In the present discussion, the focus will be upon the removal of material from one or more selected regions of a resonator structure. Unfortunately, ceramic filter components cannot be brought to a desired resonant frequency within the precision required to produce a tuned filter only by adjusting dimensions because of manufacturing spreads in the dielectric constant of the ceramic. That is to say, one manufactured batch of a ceramic material may have a dielectric constant which departs from the dielectric constant of the same material manufactured at a different time. Moreover, there may be variations in the dielectric constant within a single manufactured batch, or, indeed, within a single resonator structure. That is to say, a single resonator structure may not always be homogeneous with respect to dielectric constant. As a consequence, in order to tune the filter components and, hence, the filter overall, the resonant frequencies need to be adjusted to compensate for the dielectric constant variations. For a multimode filter component, there are multiple modes whose frequencies need to be independently adjustable. In addition, to minimize loss in the completed filter, and to maintain the design performance of the filter, the components need to be maintained as close as possible to an ideal shape.
Heretofore, ceramic components, such as monoblock ceramic filters, have been tuned by grinding off small portions of their silver coatings.
Further, the frequencies of donut ceramic resonators have been adjusted by grinding material from one face. The resonant frequency of a donut ceramic resonator is measured in a standard test cavity, then the resonator is removed for grinding, and, subsequently, it is remeasured in the test cavity. However, it is not measured in its final cavity as part of a completed filter. Because of manufacturing spreads in the final cavity, tuning screws are still needed to adjust the final filter.